1. Field of the Invention
The present invention relates generally to color cathode ray tube (CRT) indicators, such as full color indicators for displaying flight information to the pilots of an aircraft. More particularly, the invention relates to apparatus for adjusting the color purity of CRT indicators of the three cathode, shadow mask type.
2. Description of the Prior Art
As is well known, the color purity of a three gun, shadow mask type of color CRT indicators depends upon the proper angle at which the red, green and blue electron beams, as a unit, penetrate the hole pattern of the shadow mask and strike the red, green and blue phosphor triads on the internal surface of the CRT face or screen. Practical manufacturing tolerances usually prevent such properly angled penetration of the shadow mask hole pattern and color purity adjustment assemblies are usually required to compensate for such manufacturing tolerances. Such apparatus effectively repositions the electron beams, as a unit, so as to pass through the shadow mask apertures at the proper angle to provide color purity by energizing their respective red, green and blue phosphor dots without any overlap. This adjustment is usually accomplished by providing an adjustment magnetic field which reacts with the electron beam unit to slightly deflect the same to their required angle. Thus, the magnetic field must be capable of having its magnitude and its direction readily adjustable. Furthermore, normal purity adjustments are accomplished manually by an operator with the CRT energized and displaying a suitable test pattern.
Prior art purity adjustment apparatus consisted of a pair of independently adjustable ring magnets surrounding the neck of the CRT and located just forward of the three gun cathode assembly such that the resultant magnetic field of the two magnets interact with the three electron beams to effectively adjust their angular positions relative to the shadow mask and its associaed phosphor screen. Each ring magnet provides a magnetic field generally diametrically across the CRT neck and each magnet is adjustably supported on a structure fixed to the tube neck with some frictional engagement therebetween so that they remain in the positions at which they are adjusted. Normally, each magnetic is initialeach pole in opposite directions while the direction of the field may be adjusted by rotating like poles together in one direction or the other. In prior art apparatus, each ring magnet was provided with projecting tabs which could be grasped with the fingers and rotated as required. Since, as stated, such adjustments are made with the CRT operating, the operator was exposed to the dangers of very high voltage shocks. Further, in trying to adjust one of the magnetic field functions, it was very difficult to adjust the other without inadvertently readjusting the first and vice versa.
In some CRT display instruments, particularly those especially designed for use in aircraft, the CRT may be subjected to stray or uncontrollable magnetic fields which could adversely affect the quality of the display, including its color purity. Therefore, in order to insulate the CRT from such extraneous magnetic fields, the entire CRT to the rear of its display screen is surrounded by a magnetic shield, such as a low field attenuation shield made of high permeability nickel/iron sheets. Such shielding structure makes it next to impossible to adjust the purity using the prior art tabbed ring magnets.
The purity adjustment assembly of the present invention overcomes the disadvantages of the prior art assemblies and permits fully independent adjustment of magnetic field magnitude and direction without exposing the operator to dangerous shocks and permits such adjustments to be made with the CRT enclosed within the magnetic shield or other protective covering.